How Democrats are planning to fight Donald Trump's mass deportation plan

Immigration advocacy groups and Democratic leaders are seeking to disrupt President-elect Donald Trump's plan to deport millions of undocumented immigrants by pre-drafting lawsuits that could be filed as soon as he takes office.

Trump has vowed carry out what he calls "the largest deportation operation" in the country's history, and has pledged to reinstate and expand his controversial ban on people coming into the U.S. from certain majority-Muslim countries as part of his immigration policy.

On Monday, he re-emphasized on Truth Social that he is prepared to declare a national emergency and use military assets to carry out his promise of mass deportation.

Several immigration advocates and Democratic leaders told ABC News they have spent months preparing for the prospect of another Trump presidency and the expected crackdown on immigrants that Trump and his newly tapped border czar Tom Homan have promised.

Homan, who has embraced Trump's pledge to undertake mass deportations on "Day 1" of the new administration, oversaw the U.S. Immigration and Customs Enforcement agency (ICE) during the Trump administration's "zero tolerance" enforcement that separated parents from their children at the border.

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"In California, we've been thinking about the possibility of this day for months and in some cases, years, and been preparing and getting ready by looking at all of the actions Trump said he will take," California Attorney General Rob Bonta told ABC News.

Bonta said his team has prepared briefs on several immigration issues that Trump mentioned on the campaign trail. including mass deportations, birthright citizenship, Deferred Action for Childhood Arrivals (DACA) and sanctuary cities.

"There will be pain and harm inflicted by him. It is not all avoidable, but to get to our immigrant communities in ways that are in violation of the law, they're going to have to go through me, and we will stop them in courts using our legal tools given to us," Bonta said.

The California attorney general claims that 80% of the state's legal challenges against the immigration executive orders and policies from Trump's first term were successful.

"We're very confident that we will block major efforts by the federal administration, that we will be able to blunt some of the worst of it," Bonta said.

The 24 Democratic state attorneys general across the United States hope to present a unified front to block the Trump administration's immigration policy by using his first term as a blueprint, according to Sean Rankin, the president of the Democratic Attorneys General Association.

"When we look at immigration, we know that that is something that the president has talked about over and over and over again," Rankin told ABC News. "At this point, we're not connecting dots. We're following flashing arrows. It's very easy to see where they're going to go."

One of Homan's targets in his mass deportation plan are sanctuary states and cities -- places that have enacted laws designed at protecting undocumented immigrants. The policies, which vary by state, generally prohibit city officials from cooperating with the federal immigration authorities.

"They better get the hell out of the way," Homan said last week, regarding the governors of sanctuary states. "Either you help us or get the hell out of the way, because ICE is going to do their job."

Leaders in several sanctuary cities have said they are going to fight back using all the tools legally available to protect immigrant communities.

"We have been doing the work in this office to prepare for a lot of different hypotheticals and we will be prepared to face those with every tool that we have," said Washington Attorney General Bob Ferguson at a press conference last week.

Ferguson told reporters that between 2017 and 2021, his legal team defeated 55 "illegal actions" and policies from the Trump administration. But while his office has been preparing litigation for months, Ferguson said he believes the second Trump administration will also be better prepared than the first one.

"One of many reasons why we were successful with our litigation against the Trump administration was they were often sloppy in the way they rolled out and that provided openings to us to prevail," Ferguson told reporters. "In court this time around, I anticipate that we will see less of that, and that is an important difference."

In addition to considering the use of the military to carry out deportations, Trump and his allies have suggested using an obscure section of the 1798 Alien and Sedition Acts -- a set of 18th century wartime laws -- to immediately deport some migrants without a hearing.

Lee Gelernt, the deputy director of the American Civil Liberties Union's Immigrants' Rights Project, told ABC News that they have been preparing for the potential use of the military to conduct deportations.

"They're going to try and use the military, under the alien enemies act, to summarily deport people," Gelernt said. "We will try and challenge it immediately."

Gelernt, who led the ACLU's legal response to family separations in Trump's first term, said he expects the upcoming Trump administration to be "worse for immigrants" than the first.

"The Trump team has apparently been preparing for four years to implement anti-immigrant policies, and the rhetoric in the country has gotten so much more polarized than it was in 2016," Gelernt said.

During Trump's first term, Gelernt said groups like the ACLU were caught off-guard with some of his executive orders like the travel ban -- but this time around, the organization has been preparing litigation for almost a year. In 2018, the Supreme Court upheld Trump's controversial ban on travel from several predominantly Muslim countries, which the Biden administration later eliminated. Since then, Trump has appointed two Supreme Court justices.

"We are plotting out our challenges with much more advanced preparation, and we are doing our best to coordinate among all the various NGOs (non-governmental organizations) around the country," Gelernt said.

"As litigators, we've been convening, we've been preparing, we've been trying to anticipate the unimaginable as we walk into the next four years," said Alina Das, co-director of the Immigrant Rights Clinic at the New York University School of Law.